A variety of systems are used for automated inspection of semiconductor wafers, in order to detect defects, particles and/or patterns on the wafer surface as part of a quality assurance process in semiconductor manufacturing processes. It is a goal of current inspection systems to detect defects.
The electric field component of light waves is used to define the state of polarization of said waves. Polarized light waves in general are said to be elliptically polarized—there may be a certain phase shift between two perpendicular electrical field components Ex and Ey, Ex and Ey being the components of the electric field directed along the x and y directions, respectively, wherein the direction of propagation is in the z direction. The polarization state is often described by two parameters related to the ellipse generated by the electric field components: ε (the degree of ellipticity) and α (the orientation angle of the major axis). When the phase shift between the two electric field components is k*π radians (k being an integer) the light wave is linearly polarized, ε=0 and α=arc tan(cos(kπ)·Ey/Ex). When both components have the same amplitude but are phase shifted in relation to each other by (k+1/2)*π radians the light wave is said to be circularly polarized, and ε=1 (in this case α is meaningless). For intermediate polarization states, ε is between 0 and 1 and α ranges between 0 and π.
The optical properties of anisotropic materials depend on the polarization as well as the direction of propagation of light waves. The propagation of electromagnetic waves through anisotropic materials is known in the art and is described in “Optical Waves in Crystals”; A. Yariv, P. Yeh, 1984 John Wiley & Sons, Inc. pages 69–120.
Birefringence is a property of an anisotropic material that has two different indices of refraction for light polarized in different directions (said directions known as the ordinary axis and extraordinary axis), to split a light wave into an ordinary component and to an extraordinary component, said components corresponding to the components of the incident electric field along the respective axes. Accordingly, a birefringent material may produce a phase shift between the two polarization components of a light wave and may also introduce a relative amplitude shift, thus it may change the state of polarization of a light wave. The phase shift between the ordinary component and the extraordinary component is termed phase retardation. Any change of either ε or α between the incident and reflected or transmitted light will be termed forthwith as a polarization shift.
A half wave plate is an optical element that produces a phase retardation of π radians between the ordinary and extraordinary axes. Rotating a half wave plate such that one of said axes is set at an angle θ relative to the polarization direction of a linearly polarized light wave will, upon passage through the element, rotate the polarization direction of the light wave by 2θ, given that the incident beam is perpendicular to the plane of the half wave plate. Accordingly, a half wave plate that is rotated by π/4 radians relative to the incident polarization direction may convert a vertically polarized wave light to a horizontally polarized wave light.
A quarter wave plate is an optical element that produces a phase retardation of π/2 radians between the ordinary and extraordinary axes. Quarter wave plates that are oriented such that one of said axes creates an angle of π/4 radians relative to the polarization direction of a linearly polarized light wave will, upon passage through the element, convert the linearly polarized light into a circular polarized light; conversely, circularly polarized light which is perpendicularly incident on a quarter wave plate is converted to linearly polarized light with the polarization vector along an angle of π/4 radians relative to the ordinary axis.
A linear polarization filter is an optical element that transmits a particular linearly polarized component of an incident light beam, and blocks the component orthogonally polarized to said linear polarization of said light beam.
Any arbitrary elliptical polarization state of a light beam may be established through suitable application of a polarization filter, half wave plate and quarter wave plate: First the linear polarization filter is applied to the incident beam to establish a well-defined linear polarization, then the half wave plate and quarter wave plate are applied to the light beam in order to convert said linear polarization to the desired elliptical polarization. This process is termed forthwith as establishing the polarization state of the light beam.
Any arbitrary elliptical polarization component may be removed from a light beam through suitable application of a half wave plate, quarter wave plate and polarization filter. First the half wave plate and quarter wave plate are applied to the light beam in order to convert the desired elliptical polarization component into linear polarization, then the polarization filter is applied to the light beam to block said linear polarization. This process is termed forthwith as filtering the light beam by polarization.
U.S. Pat. No. 5,333,052 of Finarov describes a system and method that uses an obliquely illuminated, linearly polarized light beam to provide a contrast image, whereas the reflected light is filtered by polarization in response to the birefringence of a certain material of an inspected object. The system and method are used for detection of the presence of, or thickness variations in, transparent films.